This description relates to retroreflectors, which are optical components that retro-reflect an input light beam to produce an output light beam propagating parallel to the input beam but in an opposite direction. An example of a retroreflector is a cube corner having three optical surfaces that intersect at right angles. Retroreflectors can be used in distance measuring interferometers. For example, they can be used in multi-axis interferometers of semiconductor lithography machines to derive an angle based on the difference of two parallel distance measurements having a known separation.
In a conventional multi-axis interferometer, individual retroreflectors are positioned and mounted to achieve the desired spatial separation of the measurement axes. This process can be difficult to perform in mass production in a way that is cost effective. The individual retroreflectors may be placed in position and fastened to a frame or optically attached to another component of the interferometer. The position may have some variation from the ideal position, and there can mechanical instability between retroreflectors. To position each retroreflector, some form of metrology feed back is required. In one example, the feedback is the location of the center of a beam of light. There is a level of uncertainty related to the center of a beam of light. This uncertainty can be greater than required. The final assembly may have inherent stability issues that can result in relative motion between retroreflectors.